S. Shimada, H. Tanaka, K. Hasebe, N. Hayashi, Y. Ochi,
T. Matsui, I. Nishizaki, Y. Matsumoto, Y. Tanaka,
H. Nakamura, Y. Mizuno✉ and K. Nakamura

rubber sheet was placed at the bottom of the CFRP sheet to absorb
the ultrasonic oscillations. The transmitted light was guided to a photodetector, and its electrical output was monitored using an oscilloscope;
this conﬁguration enabled the transmitted light power measurements tobe performed in real-time during the welding process. The change in the
cross-sectional shape of the POF during the welding process was simultaneously monitored using a high-speed camera at 1000 fps.

The ultrasonic welding of polymer optical ﬁbres (POFs) onto carbonﬁbre-reinforced polymers for advanced ﬁbre-optic sensing is demonstrated. The relationships among the welding time, preload, opticalloss, and adhesive force are fully evaluated. A high-speed camera tomonitor changes in the cross section of the POF during the ultrasonicwelding is also used.

Principle and setup: Ultrasonic welding is one of the most popularmethods for joining thermoplastics because of its high-speed, low-cost
operation and ease of automation [8]. To give a typical example, ultrasonic oscillation at several dozen kilohertz is applied to stacked sheets ofthermoplastics under some preload. The boundaries of the plastics thengenerate frictional heat, leading to melting and welding [8]. This technique
is suitable for POFs with relatively low glass-transition temperatures(∼100 °C) [6] and is not applicable to glass optical ﬁbres.
In the experiment, we used POF samples with a core diameter of
0.98 mm, a cladding diameter of 1.0 mm, and an optical propagation
loss of 0.25 dB/m at a wavelength of 650 nm. The samples weremainly composed of polymethyl methacrylate, which is a thermoplastic
material. In contrast, the base material of the CFRP sheet samples(0.85 mm thick and fabricated by VaRTM) was epoxy resin, which is
known as a thermoset plastic; thus, the CFRP samples themselves did
not melt. However, melted POFs were so ﬁrmly welded to the roughsurface of the CFRP samples that the POFs precisely followed the deformation (strain and/or bending) of the CFRPs.
During ultrasonic welding of the POF, additional optical loss is inevitably induced according to the change in the cross-sectional shape of theﬁbre core. Therefore, the transmitted light power was plotted as a function of the welding time (deﬁned as the time after the ultrasonic oscillation was ﬁrst applied). The experimental setup is depicted inFig. 1a. A semiconductor laser with a wavelength of 641 nm and an
output power of 1.5 mW was used as a light source. We employed aLangevin-type transducer with a cylindrical horn and a ﬂat tip (tip diameter: 30 mm; oscillation frequency: 29 kHz; oscillation amplitude:
31 μm), the position of which was ﬁxed after the initial preload was
set to a desired value (veriﬁed using a weight scale). A silicone

force
gauge

PZT
transducer
with horn

preloadtweezers
641 nm
laser

POF

PD

CFRP
silicone

OSC

POF
CFRP

a

b

Fig. 1 Schematic setup
a Setup for ultrasonic welding of POFs. AMP: ampliﬁer; CFRP:
carbon-ﬁbre-reinforced polymer; FG: function generator; OSC: oscilloscope;
PD: photo detector; PZT: lead zirconate titanateb Setup for measuring the POF-to-CFRP adhesion force

The POF-to-CFRP adhesion force was also evaluated. First, 100 mm
long POF samples were welded onto the CFRP sheets under different
preloads (welded length: ∼30 mm; welding time: 1 s). Then, as shownin Fig. 1b, the welded POFs were nipped with tweezers (tip width:∼5 mm) that were connected to a force gauge, which was graduallylifted upward. In this evaluation, the POF-to-CFRP adhesion force
was deﬁned as the force at which the welded bottom of the POF
began to peel.
Experimental results: Fig. 2a shows the transmitted light power as afunction of the welding time. The initial preloads were varied from 3
to 20 N. The vertical axis was normalised so that the optical power
under preload before welding was 1. As the preload increased (resulting
in a larger deformation of the POF), the optical power tended to decrease
rapidly. To quantitatively evaluate this behaviour, we plotted the preload
dependence of the welding time required for the normalised transmitted
power to decrease to 0.8 (Fig. 2b). We found that the required welding
time exponentially decayed as the preload increased. Note that, inFig. 2a, the transmitted power ultimately decreased to 0 for all the preloads. By repeated experiments, we found that this was either because
the cross-section of the POF was completely crushed (for relatively
high preloads) or because the POF was cut (for relatively low preloads).
The latter phenomenon can be explained as follows. As the POF melted,
a gap was generated between the horn and the POF; then, owing to thedeﬂection of the POF, the ultrasonic oscillation was locally applied to acertain point of the POF. This mechanism can also explain the unstablebehaviour that is observed after 5 s of welding for relatively low
preloads (see Fig. 2a).

normalised power, a.u.

Introduction: The age-induced degradation and seismic damage of steel
structures have recently become a serious social problem. One solution
to this problem involves utilising carbon-ﬁbre-reinforced polymers(CFRPs) – composite materials that offer a high strength-to-weightratio and rigidity – as reinforcing materials [1–4]. CFRPs can be fabricated in various ways. One low-cost fabrication technique isvacuum-assisted resin transfer moulding (VaRTM) [2, 3], wherein theresin is infused into dry fabrics formed on a mould under vacuum
pressure. There exists a considerable demand for effective healthmonitoring techniques of the CFRP-reinforced components [3, 4].
One of the most promising monitoring techniques is ﬁbre-optic
sensing with distributed strain and temperature measurement capability[5]. As the glass optical ﬁbres commonly used in such sensors are relatively easily damaged, an increasing amount of attention has been paid
to the use of plastic optical ﬁbres (POFs) [6, 7], which possess muchlarger core diameters (up to ∼1 mm) and extremely high ﬂexibilities.
Accordingly, the use of POFs in the ﬁeld is signiﬁcantly more practical,
as they pose a much lower risk of ﬁbre breakage. Conventionally,adhesive materials (such as epoxy glue) are often used to ﬁx the
POFs onto the surface of the CFRPs, but this method poses two
problems: (i) the waiting time required for the adhesive to harden islong (several minutes or longer), and (ii) the deformation of the
CFRP, which should be detected by the ﬁbre sensor, becomes absorbedby the adhesive layer.
To mitigate these problems, in this study we develop a new techniquefor ﬁxing the POFs onto the surface of the CFRPs, which is based on
ultrasonic welding [8, 9]. In our experiment, the POFs are successfully
welded onto the CFRP surfaces within a few seconds. We then measure
the optical propagation loss of the POFs as a function of welding time,and investigate the adhesive force as a function of the preload.

pull

AMP
FG

1.0

preload (N)

0.8

20

8

0.6

15

5

10

3

0.4

3.0
2.5
2.0
time, sUltrasonic welding of polymer optical fibres
onto composite materials

0.2

1.5
1.0
0.5
.0

.0
0

5

10
time, s10
15
preload, N

a

b

15

0

5

20

Fig. 2 Measured resultsa Normalised transmitted light power as a function of welding time under
different initial preloads
b Relationship between the preload and the time required for the normalised
power to decrease to 0.8

We then observed the change in the cross section of the POF during
the ultrasonic welding using a high-speed camera. The initial preload
was set to 8 N. Fig. 3a shows the normalised transmitted light power
as a function of the welding time t; the photographs taken at the corresponding welding times (t = 0, 0.3, 0.6, and 1.0 s) are shown in Figs. 3b–e,
respectively. Only the portions of the POF that touched the CFRP
surface were observed to melt. This is because the friction coefﬁcient

ELECTRONICS LETTERS 18th August 2016 Vol. 52 No. 17 pp. 1472–1474

normalised power, a.u.

at the boundary between the POF and the rough surface of the CFRP
was larger than that at the boundary between the POF and the smoothsurface of the transducer horn. This behaviour is preferable to the undesired increase in optical loss.
1.0
A
0.8

B

D

preload
=8 N

1.0

1.2

C

POF-to-CFRP adhesive force were then quantitatively evaluated, andwe found that there exists a trade-off relationship between these twoquantities. The adhesive force can be drastically enhanced either by
the multipoint/continuous welding of the POF or by the use of special
CFRPs, the base materials of which are comprised of thermoplastics.
We anticipate that this technique can also be used to ﬁx and embed
the POFs in the CFRPs, especially during the VaRTM process, andthat these results will be useful in applying POF-based sensing technology
to the health monitoring of composite materials.

0.6
0

0.2

0.4

0.6
0.8
time, s1.4

aAcknowledgments: This work was supported by MLIT Construction
Technology Research and Development Subsidy Program and by
JSPS KAKENHI Grant Numbers 25709032, 26630180, and 25007652.

B : 0.3 s

A : 0.0 s
horn© The Institution of Engineering and Technology 2016
Submitted: 18 March 2016 E-ﬁrst: 22 July 2016
doi: 10.1049/el.2016.0905
One or more of the Figures in this Letter are available in colour online.

POF
1 mm

1 mm

CFRP
siliconeb

S. Shimada, H. Tanaka, K. Hasebe, Y. Mizuno and K. Nakamura(Laboratory for Future Interdisciplinary Research of Science andTechnology, Tokyo Institute of Technology, Yokohama 226-8503,
Japan)c
D : 1.0 s

C : 0.6 s✉ E-mail: ymizuno@sonic.pi.titech.ac.jp
1 mm

N. Hayashi (Research Center for Advanced Science and Technology,
The University of Tokyo, Tokyo 153-8904, Japan)

1 mm

d

Y. Ochi (Advanced Composites Center, Toray Industries, Inc., Nagoya
455-0024, Japan)eFig. 3 High-speed monitoring resultsa Normalised transmitted light power as a function of welding time under an 8 N
preload (magniﬁed view of Fig. 2a)b–e High-speed micrographs of the ultrasonically welded POF at 0.0, 0.3, 0.6, and
1.0 s, respectively

Finally, the adhesive force of the welded POF was measured withrespect to the preload (Fig. 4). For each preload, the same measurement
was performed 5 times, and the average values were plotted with error
bars that represent their standard deviations. For low preloads of<5 N, the POF was not welded onto the CFRP surface. As the preload
increased, the adhesive force increased; this behaviour is valid, considering that the higher preload induces larger deformation of the POF (at t =1 s in this measurement), leading to a larger melted area and a more
complete adhesion onto the rough surface of the CFRP. Thus, a trade-off
relationship between the optical loss and the adhesive force was experimentally veriﬁed. Note that the adhesive force can be signiﬁcantly
augmented by welding multiple points of a single POF. The continuous
welding of a single POF in the longitudinal direction could thus lead to ahigh adhesive force.
5

force, N

4

3

2

1

0
0

5

10

15

20

preload, N

Fig. 4 Adhesive force as a function of the preload (welding time t = 1 s)

Conclusion: By exploiting the ultrasonic welding technique, we
demonstrated a new procedure for rapidly ﬁxing POFs onto CFRP surfaces. The time required for the POF welding was on the order of
seconds, which was far shorter the time associated with conventional
methods. The optical propagation loss of the POFs and theT. Matsui (First Advanced Composites Technical Department, Toray
Industries, Inc., Nagoya 455-0024, Japan)
I. Nishizaki (Innovative Materials and Resources Research Center,
Public Works Research Institute, Ibaraki 305-8516, Japan)
Y. Matsumoto (Department of Architecture and Civil Engineering,
Toyohashi University of Technology, Aichi 441-8580, Japan)
Y. Tanaka (Institute of Engineering, Tokyo University of Agriculture
and Technology, Tokyo 184-8588, Japan)
H. Nakamura (Department of Civil and Environmental Engineering,Tokyo Metropolitan University, Tokyo 192-0397, Japan)
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